Thermal Martensitic Transformation Research Articles

Structural metamaterial lattices by laser powder-bed fusion of 17-4PH steel

Additive manufacturing build parameters are used to engineer structural metamaterials lattices with controllable mechanical performance, achieved through microstructural grading of 17-4PH steel without compositional or geometric modification. The high solidification rates of laser powder-bed fusion suppress the thermal martensitic transformation and lead to elevated levels of retained austenite. Diamond cubic lattices built at low energy density (low thermal strain) retain a low martensite phase fraction (3 wt%) and exhibit a bend-dominated compression response. Lattices built at high energy density experience increased thermal strain during the build, causing in-situ deformation-driven transformation, yielding 44 wt% martensite; these exhibit a stretch-dominated compression response. Metamaterial lattices, with high and low energy density parameters in different configurations, exhibit mixed compression responses. Controllable mechanical response was achieved through control of microstructure, using build parameters to adjust thermal strain and selectively suppress or trigger the martensitic phase transformation in-situ.

Designing for Shape Memory in Additive Manufacturing of Cu-Al-Ni Shape Memory Alloy Processed by Laser Powder Bed Fusion.

Shape memory alloys (SMAs) are functional materials that are being applied in practically all industries, from aerospace to biomedical sectors, and at present the scientific and technologic communities are looking to gain the advantages offered by the new processing technologies of additive manufacturing (AM). However, the use of AM to produce functional materials, like SMAs, constitutes a real challenge due to the particularly well controlled microstructure required to exhibit the functional property of shape memory. In the present work, the design of the complete AM processing route, from powder atomization to laser powder bed fusion for AM and hot isostatic pressing (HIP), is approached for Cu–Al–Ni SMAs. The microstructure of the different processing states is characterized in relationship with the processing parameters. The thermal martensitic transformation, responsible for the functional properties, is analyzed in a comparative way for each one of the different processed samples. The present results demonstrate that a final post–processing thermal treatment to control the microstructure is crucial to obtain the expected functional properties. Finally, it is demonstrated that using the designed processing route of laser powder bed fusion followed by a post–processing HIP and a final specific thermal treatment, a satisfactory shape memory behavior can be obtained in Cu–Al–Ni SMAs, paving the road for further applications.

Characteristics of nucleation and transformation sequence in deformation-induced martensitic transformation

To elucidate the characteristics of the nucleation and transformation of deformation-induced martensitic transformation, detailed characterisations were performed during quasi-in-situ tensile deformation. Two possible transformation scenarios were simultaneously observed and investigated in detail. ε-Martensite initiated within the slip band is the most preferential nucleation site instead of slip band intersections as assumed in the classical theory. The correlation between the two transformation sequences is discussed and associated with strain and stress conditions. Characteristics of the nucleation and transformation sequence upon deformation are compared to thermal martensitic transformation; systematic similarity is discovered, implying the underlying shared nature of thermal and mechanical stabilities.

On ductile fracture of 316L stainless steels at room and cryogenic temperature level: An engineering approach to determine material parameters

Two austenitic 316L stainless steels from different provider are tested mechanically. Uniaxial tensile experiment serves the basis to determine material parameters for elastic-plastic response at different temperature levels considering from room (23 ∘ C) to cryogenic (-163 ∘ C) temperatures. The typical plastic strain induced kinetic transformation of martensite content becomes appreciable as temperature decreases yielding an apparent improvement in material strength with its irreversible loss in ductility. The evolution of martensitic content inside the austenitic matrix would increase the macroscopic strain hardening significantly and thus delaying localization by preventing through thickness necking. The present work summarizes the experimental/computational program performed on metastable austenitic 316L stainless steel under a number of different tensile stress states at low temperatures. The results indicate that at room temperature the martensite kinetic transformation is negligible and it is surpassed by slip mechanism of austenite grain matrix and a simple isotropic hardening law is sufficient to describe plastic flow. On the other hand, at low temperatures the phase transformation process of FCC γ -austenite into BCC α ’-martensite is triggered leading to a significant material strength enhancement, but with subsequent loss of ductility. A suitable constitutive model with pressure-sensitive quadratic yield function and temperature dependent hardening law that relates the increasing of yield strength to the thermal martensitic transformation kinetic process is proposed. Likewise, the Modified Mohr-Coulomb (MMC) model is then extended to incorporate thermal effects on ductile fracture initiation at different temperature levels, stress state and loading conditions.

H-Phase Precipitation and Martensitic Transformation in Ni-rich Ni–Ti–Hf and Ni–Ti-Zr High-Temperature Shape Memory Alloys

The distributions of H-phase precipitates in Ni50.3Ti29.7Hf20 and Ni50.3Ti29.7Zr20 alloys formed by aging treatments at 500 and 550 °C or slow furnace cooling and their effects on the thermal martensitic transformation have been investigated by TEM and calorimetry. The comparative study clearly reveals faster precipitate-coarsening kinetics in the NiTiZr alloy than in NiTiHf. For precipitates of a similar size of 10–20 nm in both alloys, the martensite plates in Ni50.3Ti29.7Zr20 have larger widths and span a higher number of precipitates compared with the Ni50.3Ti29.7Hf20 alloy. However, for large H-phase particles with hundreds of nm in length, no significant differences in the martensitic microstructures of both alloy systems have been observed. The martensitic transformation temperatures of Ni50.3Ti29.7Hf20 are ~ 80–90 °C higher than those of Ni50.3Ti29.7Zr20 in the precipitate-free state and in the presence of large particles of hundreds on nm in length, but this difference is reduced to only 10–20 °C in samples with small H-phase precipitates. The changes in the transformation temperatures are consistent with the differences in the precipitate distributions between the two alloy systems observed by TEM.

Toward an adaptive vibration absorber using shape-memory alloys, for civil engineering applications

This article explores the capabilities of a novel adaptive vibration absorber for civil engineering structures, with regard to frequency self-tuning, based on the temperature modulation of shape-memory alloy restitution elements. This real-time temperature modulation of shape-memory alloys, through Joule effect, enables to control the elastic modulus of these elements, by inducing thermal martensitic transformations, and allows for the adaptation of the stiffness of the absorber, in order to be continuously tunable for a wide frequency range. A series of simulations are performed, using numerical models of a lively footbridge, to give an additional insight into the high potentialities of this adaptive control approach in the mitigation of vibrations in civil engineering structures.

Nano/ultrafine grained austenitic stainless steel through the formation and reversion of deformation-induced martensite: Mechanisms, microstructures, mechanical properties, and TRIP effect

A comprehensive study was carried out on the strain-induced martensitic transformation, its reversion to austenite, the resultant grain refinement, and the enhancement of strength and strain-hardening ability through the transformation-induced plasticity (TRIP) effect in a commercial austenitic 304L stainless steel with emphasis on the mechanisms and the microstructural evolution. A straightforward magnetic measurement device, which is based on the measurement of the saturation magnetization, for evaluating the amount of strain-induced martensite after cold rolling and reversion annealing in metastable austenitic stainless steels was used, which its results were in good consistency with those of the X-ray diffraction (XRD) method. A new parameter called the effective reduction in thickness was introduced, which corresponds to the reasonable upper bound on the obtainable martensite fraction based on the saturation in the martensitic transformation. By means of thermodynamics calculations, the reversion mechanisms were estimated and subsequently validated by experimental results. The signs of thermal martensitic transformation at cooling stage after reversion at 850°C were found, which was attributed to the rise in the martensite start temperature due to the carbide precipitation. After the reversion treatment, the average grain sizes were around 500nm and the nanometric grains of the size of ~65nm were also detected. The intense grain refinement led to the enhanced mechanical properties and observation of the change in the work-hardening capacity and TRIP effect behavior. A practical map as a guidance for grain refining and characterizing the stability against grain growth was proposed, which shows the limitation of the reversion mechanism for refinement of grain size.

Biomaterial Co-Cr-Mo Alloys Nano Coating Calcium Phosphate Orthopedic Treatment

The modem study a thermal martensitic transformation of biomedical Co-Cr-Mo alloys and ultimately offers large elongation to failure while maintaining high strength. In the future study, structural evolution and dislocation slip as an elementary process in the martensitic transformation in Co-Cr-Mo alloys were investigated to reveal the origin of their enhanced phase stability due to nitrogen addition and coating of calcium phosphate specimens with and without nitrogen addition were prepared. The N-doped alloys had a single-phase matrix, whereas the N-free alloys had a duplex microstructure. Irrespective of the nitrogen content, dislocations frequently dissociated into Shockley partial dislocations with stacking faults. The Nano range coating of calcium phosphate function as obstacles to the glide of partial dislocations and consequently significantly affect the kinetics of the martensitic transformation. As a result, the formation of marten site plays a crucial role in plastic deformation and wear behavior, the developed nanostructures modification associated with nitrogen addition must be a promising strategy for highly durable orthopedic implants.

Effect of Titanium Depletion of High Temperature Treated NiTi Shape Memory Alloy Wire

Shape memory behaviour of NiTi is very sensitive to the presence of inclusion in the matrix and oxide and the surface. This work studied the influence of high temperature treatment towards thermal martensitic transformation behaviour of NiTi. The alloy was subjected to heat treatment at 1223 K for various times in argon and air environment. It was found that treatment in air severely destroyed the shape memory property of the alloy due to the formation of multilayer of oxides. Treatment in argon also reduced the volume percentage of martensite phase transformation, even though no oxide layer was observed at the surface. This reduction is caused by the depletion of Ti element in the matrix due to the outward diffusion of Ti element. The outward diffusion of Ti is however stabilized at 15 hours exposure time of 1223 K heating temperature.

In situ photoelectron emission microscopy of a thermally induced martensitic transformation in a CuZnAl shape memory alloy

We report photoelectron emission microscope observations of the thermal martensitic transformation in a CuZnAl shape memory alloy. The phase transformation appears at 48°C during heating and at 42°C upon cooling. The transformation is marked by a sharp change in photoelectron intensity, as well as a significant displacement and reorientation of surface features. The difference in the photoelectron intensity before and after the transformation is attributed to a change in work function of about 0.2eV. Photoemission electron microscopy provides real-time information on microstructural changes and phase-dependent electronic properties.

Correlations between the calculated stacking fault energy and the plasticity mechanisms in Fe–Mn–C alloys

Abstract A model is proposed for the evaluation of the stacking fault energy (SFE) in Fe–Mn–C austenitic alloys, at different temperatures. It accounts for the variation of the Gibbs energy of each element during the austenite to e martensite transformation, plus their interactions. The Gibbs energy due to the antiferromagnetic to paramagnetic transition is also taken into account. The required data have been obtained from the literature. The result shows a decrease of the SFE with temperature, with a saturation below the austenite Neel temperature. The result agrees with the mechanical and thermal martensitic transformation limits proposed by Schumann. The plasticity mechanisms depend on the SFE. The mechanical martensitic transformation occurs below 18 mJ/m 2 , and twinning between 12 and 35 mJ/m 2 , in agreement with the tensile tests and the deformation microstructures observed in an Fe–22 wt.% Mn–0.6 wt.% C alloy at 77, 293 and 693 K.

Coupled stress-strain and electrical resistivity measurements on copper based shape memory single crystals

Recently, electrical resistivity (ER) measurements have been done during some thermomechanical tests in copper based shape memory alloys (SMA's). In this work, single crystals of Cu-based SMA's have been studied at different temperatures to analyse the relationship between stress (s) and ER changes as a function of the strain (e). A good consistency between ER change values is observed in different experiments: thermal martensitic transformation, stress induced martensitic transformation and stress induced reorientation of martensite variants. During stress induced martensitic transformation (superelastic behaviour) and stress induced reorientation of martensite variants, a linear relationship is obtained between ER and strain as well as the absence of hys teresis. In conclusion, the present results show a direct evidence of martensite electrical resistivity anisotropy.

Magnetic properties of stress-induced martensite and martensitic transformation in Ni–Mn–Ga magnetic shape memory alloy

Thermoelastic and strain-induced martensitic transformations of single crystalline Ni 49.7 Mn 29.1 Ga 21.2 magnetic shape memory alloys were investigated by simultaneous detection of the strain and magnetization. The transformation temperature to austenite was T A =314 K. The thermoelastic transformation was shifted to higher temperature with increasing compressive stress with rate about 4 MPa/K. The effect of the moderate magnetic field up to 1 T on transformation was negligible. Very similar magnetic properties of thermally and stress-induced martensites measured in the field up to 1.2 T suggest an equivalence of the phases. Direct comparison of the magnetic properties of austenite and stress-induced martensite at constant temperature above the thermal martensitic transformation is made. At 317 K, the stress-induced martensite had a large uniaxial magnetic anisotropy, K u =1.2×10 5 J/m 3 and the saturation magnetization increased, about 14%, compared with the parental austenite.

Strains induced by magnetic field and phase transformation in Ni50.5Mn26.2Ga23.4 ferromagnetic shape memory alloy

In recent years, a lot of investigations have been focused on ferromagnetic shape memory alloys (FSMAS), such as Ni2MnGa, FePd, CoNi and FeNiCoTi [1–4], due to their large magnetic-field induced strain (MFIS) and high response frequency. Of the above alloys Ni Mn Ga alloy is the only known Heusler alloy exhibiting thermal martensitic phase transformation. It is reported up to now that a maximum 6% of MFIS has been obtained in Ni49.8Mn28.5Ga21.7 single crystal by applying a constant 2 MPa compression stress [5]. However, few results have been reported about the temperature dependence of MFIS in NiMnGa alloy. The purpose of the present paper is to measure the MFIS along the [110] direction of Ni50.5Mn26.2Ga23.4 single crystal at different temperatures. It is interesting to find that the MFIS between the temperature of Ms and Mf is larger than the MFIS below Mf. The specimen used in this study was cut from a bulk single crystal with composition of Ni50.5Mn26.2Ga23.4 (at%). The single crystal was grown in the [110] direction according to the cubic parent phase using a Czochralski instrument with a cold crucible system. This single crystal was annealed at 850 ◦C for 12 h, then quenched into ice water to obtain high L21 ordering [6]. An orientation along the specimen length direction was confirmed to be [110] by Laue X-ray diffraction. The dimensions of specimens for MFIS measurements were 2 mm × 4 mm × 6 mm. The magnetic field induced strain was measured by a strain-gauge attached to the sample which was placed in a furnace between the electromagnets. The external magnetic field was applied parallel to the [110] direction of the sample. The sample was cooled from high temperature to a temperature far below Mf. Then the MFIS was measured at the temperatures of interest. The phase transformation temperatures were determined by a low field ac susceptility method. Fig. 1 shows the χ–T curve of the experimental alloy. It can be seen that, the martensitic transformation start temperature (Ms) and finish temperature (Mf) are 279 K and 274 K respectively. And the reverse transformation temperature (TA) is 283 K. The Curie temperature (Tc) is about 370 K higher than the martensitic transformation and reverse transformation temperatures indicating that both autensite and martensite are ferromagnetic in this alloy. It is seen that the value of Ms–Mf is very small which means that thermal martensitic transformation happens quickly. Fig. 2 shows a typical curve of the MFIS measured along the [110] direction in a reverse magnetic field at 244 K below Mf. It was found that the sample shrinks in the external magnetic field. A small MFIS, about −600 ppm, was obtained with an axial magnetic field up to 2 T. Further, it can been seen the MFIS is reversible and reaches saturation at about 0.8 T. All strain recovers after returning the field to zero. Similar MFIS curves are obtained irrespective of the direction of the applied magnetic field. Fig. 3 depicts the magnetic field induced strains as a function of magnetic field in the temperature range between 244 K and 276 K. According to the different phase states at various temperatures, the experimental results can be divided into two temperature stages: Stage I. Full martensite state which is below Mf. For the specimens tested at 247 K, 255 K and 263 K, the saturated MFIS is only−600 ppm. And with the increasing of temperature only an increase of 100 ppm is observed. In this stage, all the MFIS is attributed to the motion of martensite twin boundaries caused by the magnetic field because only martensite phase exists. It is known that [110] is the hard axis of magnetization for NiMnGa alloy. According to R.C. O’Handley’s model [6] the experimental Ni Mn Ga alloy has weak anisotropy. This means that only a weak driving force could be provided on the twin boundaries regardless of the strength of applied field, which leads to very a small MFIS in the full martensite phase. Stage II. Parent phase and martensite co-exist which is between Ms and Mf. For the specimens tested at 275 K, the strain produced is as large as −900 ppm, meanwhile, of the total strain about −300 ppm strain does not recover when the magnetic field is removed. At 276 K, the saturated MFIS jumps abruptly to −1700 ppm, which is nearly three times as large as that of the specimen measured below Mf, and a greater irreversible MFIS, nearly −1200 ppm, is observed. Clearly, the strain caused by thermal phase transformation plays an important role in the increase of this MFIS. Based on the analysis of experimental results, the temperature dependences of saturated MFIS and the strain recovery

Stabilisation of martensite by applying compressive stress in Cu-Al-Ni single crystals

Compression tests have been carried out for a Cu-Al-Ni single crystal at temperatures well above the martensitic transformation, near the transformation or below it (in martensitic state). The composition was selected in order to obtain either β-β′ or β-γ′ thermal martensitic transformations, after suitable thermal treatments. The characteristics of the martensitic transformation and structural changes after the compression tests have been studied by means of calorimetry (DSC) and TEM. The obtained results show that when a compressive stress is applied on quenched samples (TTA treatment, β-β′ thermal transformation) a β-(β′)-γ′ transformation or a β′-γ′ one are stress-induced, depending whether the initial state is the parent or the martensitic phase. For aged samples (TTB treatment, β-γ′ thermal transformation) the application of stress brings about the β-γ′ transformation or γ′ re-orientation, depending on the initial state. In all the cases a notable martensite stabilisation is observed only when the stress–strain loop is not closed, that means when a permanent strain remains in the material after unloading. A direct relationship between the applied deformation when stressing the sample and the degree of stabilisation has been obtained for different strain values (between 5% and 12%) and for each set of samples (TTA and TTB). At the same time, the evolution of the characteristics of the martensitic phases with the degree of deformation has been studied. The stress induced stabilisation mechanism is related to the presence of non-twinned γ′ martensite which makes difficult the retransformation to the parent phase.

Development of shape-memory alloy actuators. Performance assessment and introduction of a new composing approach

A servo actuator using shape-memory alloy (SMA actuator), which performs special functions not found in conventional actuators, is expected to be a new driving source for future robots. This paper evaluates its performance and discusses the fundamental issues in designing such actuators. Being driven by the thermal martensitic transformation of the SMA itself, an SMA actuator is a driving system which can in principle, perform without any frictional parts, can be designed at a high output/weight ratio especially when a light-weight actuator is to be constructed But has low energy efficiency due to its mechanical characteristics of operating at a small temperature difference, and has a relatively low response. Our study shows some ways of overcoming these disadvantages by a special design approach. The method is called ξarray and is characterized by SMA wires being connected in parallel structurally and in series electrically. This approach enables the response to be enhanced due to the increased surface a...